The first observation of a solar flare in the 3- to 7-terahertz (THz) range has been reported by researchers at Mackenzie Presbyterian University’s Radio Astronomy & Astrophysics Center (CRAAM-UPM) in São Paulo, Brazil, in collaboration with colleagues in Brazil and elsewhere.
The feat was announced at the annual meeting of the American Astronomical Society’s Solar Physics Division in Boulder, Colorado, on May 31 to June 3.
“We succeeded in proving that solar flares can be detected in the terahertz range. This opens up new observational perspectives,” said Pierre Kaufmann, a researcher at CRAAM-UPM and principal investigator for the project, in an interview with Agência FAPESP.
The observation was made using the space experiment Solar-T, a double photometric telescope designed and built in Brazil by researchers at CRAAM-UPM in collaboration with colleagues at the University of Campinas’s Center for Semiconductor Components (CCS-UNICAMP).
Developed with support from FAPESP via a Thematic Project and a regular research grant, Solar-T was coupled to a stratospheric balloon launched on January 19 by NASA, the US National Aeronautics & Space Administration, from McMurdo, the main US station in Antarctica, on a mission to study the Sun (read more at agencia.fapesp.br/22721).
During its 12-day circumnavigation flight at an altitude of 40,000 m over Antarctica, Solar-T uninterruptedly collected the energy from solar flares at 3 THz and 7 THz, corresponding to a fraction of far-infrared radiation.
Solar flares are best observed in the terahertz range of the electromagnetic spectrum, which lies between visible light and radio waves. Observations in this frequency band enable scientists to understand more about the explosions created when magnetic fields in active regions of the Sun suddenly change, often hurling jets of accelerated negatively charged particles (electrons) at very high speeds toward Earth.
Far-infrared radiation from solar flares can be used in a new approach for the investigation of phenomena that produce energy in active regions located in the three layers of the Sun’s atmosphere: the photosphere, its visible surface, where temperatures reach 5,700 degrees; the chromosphere (20,000 degrees); and the corona (more than 1 million degrees).
The problem, Kaufmann explained, is that these terahertz frequencies cannot be measured at ground level because they are blocked by Earth’s atmosphere. “We have to go into space to measure them. So a new technology had to be developed for detection at these very high frequencies,” he said.
Thanks to Solar-T, the researchers were at last able to observe a solar flare in the 3 THz-7 THz range.
On January 28, at precisely 12:12:10 (GMT), the photometric telescope observed an impulsive solar energetic particle (SEP) event at 3 THz and 7 THz. Impulsive solar events have fast onsets and short durations.
The event coincided with an impulsive flare detected at 0.2 THz-0.4 THz by the Solar Submillimeter Telescope (SST), operated by UPM at these frequencies at El Leoncito Astronomical Complex in the Argentinian Andes.
Simultaneous flares were observed at the same time on the same day as a brightening in the visible light spectrum (specifically the hydrogen-emission line known as H-alpha red light) by the HASTA Telescope, also located in Argentina, and in extreme ultraviolet (EUV) by NASA’s Solar Dynamics Observatory (SDO).
“From the EUV observation, you can see that before the solar flare begins, a large structure is formed, consisting of a magnetic arch with a bright tip that falls toward the sunspot,” Kaufmann said. “The moment when the bright tip of the magnetic arch hits the surface of the sunspot coincides exactly with the impulsive solar flare detected at 3 THz-7 THz, 0.2 THz-0.4 THz, and in the visible light spectrum.”
According to Kaufmann, many members of the radio astronomy and astrophysics communities doubted that it was possible to detect solar flares in the 3 THz-7 THz range. In the preceding ten years, he and his team had already observed solar flares in the 0.2 THz-0.4 THz range, and in the preceding four years, they had succeeded in monitoring solar flares at 30 THz (mid-infrared) using a telescope also located in Argentina and another on the roof of one of Mackenzie Presbyterian University’s buildings in downtown São Paulo.
However, Kaufmann went on, solar flares observed at 30 THz were believed to be of a different nature from those observed at 0.2 THz-0.4 THz, and no one knew if solar flares occurred between these frequency bands, let alone if they could be observed.
“Now that we’ve detected solar flares at 3 THz and 7 THz, there can no longer be any doubt that solar flares exist at these frequencies, that they can be observed, and that their intensity rises in line with the rise in frequency, which was another oft-posed question,” he said.
“Apparently, solar flares at 3 THz and 7 THz are relatively intense for very weak signals and are also observed at lower frequencies, in the range of 0.2 THz-0.4 THz, although they’re well detected there.”
Detection at 3 THz and 7 THz will have implications for the interpretation of the phenomenon’s mechanisms, such as whether they are the same as the well-known mechanisms of solar flares observed at lower frequencies, he added.
The work also opens up new prospects for the observation of solar flares on two fronts. One will be the increase in observation frequencies made feasible by the new High Altitude Terahertz Solar Telescope (HATS) to be installed in an observatory at an altitude of 5,500 m near Famantina in the Argentinian Andes.
Originally, HATS was to operate at 0.85 THz and 1.4 THz. “However, we’ve decided to operate it at even higher frequencies than those originally planned,” Kaufmann said.
Another prospect for future detection is the installation of an improved version of Solar-T, capable of detecting events in a broader range of frequencies, in the Russian module of the International Space Station (ISS).
Mackenzie Presbyterian University (UPM) has a cooperation agreement with Moscow’s Lebedev Physics Institute to develop terahertz telescopes for installation on the ISS. The success of the Solar-T mission was a precondition for the agreement to proceed, demonstrating qualification of the technology developed by the Brazilian researchers.
“The telescope to be installed on the ISS will probably have five or six different terahertz frequencies,” Kaufmann said. “We plan to extend coverage of the terahertz solar flare detection spectrum as far as possible.”